X86 uses ISD::isBuildVectorAllZeros to peek through bitcasts to match zero vectors in a large number of combines.

Ideally we'd use SDPatternMatch to match these combines and use m_Zero() instead, but regrettably this fails to match the zero vectors through bitcasts,

84397c6 shows an example of the regression

If we convert m_Zero() to something closer to m_AllOnes / AllOnes_match we might be able to make this work.